In recent years, along with a developing technique of recording a motion picture, still picture and other video data in digital, large volumes of data have come to be handled, optical disk devices, such as CDs (compact disks) and DVDs (digital versatile disks), have drawn attentions as high-capacity recording devices, and studies on attaining a further higher-capacity has been pursued.
FIG. 1A is a schematic perspective view showing a state of irradiating a light on a rewritable type optical disk according to a conventional example.
An optical disk DC has a disk shape having a thickness of 1.2 mm or so, on which a center hole CH is opened at the center portion, and driven to rotate in the driving direction DR.
When recording or reproducing information, an optical recording layer in the optical disk DC is irradiated with a light LT, such as a laser light, from an objective lens OL.
FIG. 1B is a schematic sectional view showing a state of irradiating a light and corresponds to a sectional view along the line A-A′ in FIG. 1A. Also, FIG. 1C is an enlarged sectional view of a key portion.
On one surface of a medium substrate 15 made by a polycarbonate resin, etc. having a thickness of 1.1 mm or so is provided with an uneven shape including a continuous groove shaped concave portion 15d formed, for example, in spirals. On the surface thereof, an optical recording layer 16 made by a stacked structure obtained by stacking, for example, a reflection film, a dielectric film, a recording film, a dielectric film, etc. in this order. The configuration and the number of the layers of the optical recording layer 16 differ depending on a kind of a recording material and a design, and cases of a single layer configuration and a multilayer stacked structure configuration are included.
The above recording film is a recording film including, for example, a phase change type recording film, magneto-optical recording film or an organic dye material.
Furthermore, a protective film 17 having a film thickness of 0.1 mm or so is formed on the optical recording layer 16.
When recording or reproducing the above optical disk, a recording or reproducing light LT, such as a laser light, is converged by an objective lens OL and irradiated to the optical recording layer 16 from the medium substrate 15 side.
When reproducing the optical disk, a return light reflected on the optical recording layer 16 is received by a light receiving element, a predetermined signal is generated by a signal processing circuit, and a reproduction signal is taken out.
In the above optical disk, the optical recording layer 16 has an uneven shape in accordance with an uneven shape including the continuous groove shaped concave portion 15d provided on one surface of the medium substrate 15, and the uneven shape divides a track region to lands and grooves and is used as a tracking guide groove when recording and reproducing.
There are a land-groove recording method for recording information both on the lands and grooves and a recording method of using only one of the lands and grooves as a recording region.
Also, by making the uneven shape of the above medium substrate 15 a successive pit array having a length corresponding to recording data, and providing a reflection film of an aluminum film, etc. as the above optical recording film 16, a read-only-memory (ROM) type optical disk can be also obtained.
A method of producing the above optical disk will be explained.
First, as shown in FIG. 2A, for example, on a glass substrate 10 polished to be flat and washed as a substrate of a master disk for mastering, a photoresist material, which becomes alkali-soluble by being exposed, is applied to be a predetermined film thickness of 50 to 100 nm or so to form a resist film 11, so that a resist master disk RD is formed.
Next, as shown in FIG. 2B, for example, by converging and irradiating a light for exposing the resist film on the resist film 11, moving the exposure light in the radius direction of the glass substrate 10 while rotating the glass substrate 10, and exposing by an uneven shaped pattern for transferring to the medium substrate, for example, in spirals, an exposed resist film 11b and unexposed resist film 11a are obtained.
Next, as shown in FIG. 2C, the resist film 11 is developed by an alkali developing solution. As a result, the exposed resist film 11b elutes and only the unexposed resist film 11a remains, so that an uneven shaped concave portion 11d in spirals is formed on the glass substrate 10 and the resist film 11a. 
From the above, a master disk for mastering is obtained.
Next, as shown in FIG. 3A, the master disk for mastering obtained above is subjected to plating processing, etc. with nickel or other material to be a thickness of, for example, 0.3 to 0.5 mm, so that a master stamper 12 is formed.
An electric plating method featuring a rapid growth rate in plating is used for the nickel plating processing, but since it is necessary that the surface of the master disk for mastering has to be conductive in advance, a nickel thin film has to be coated by a sputtering method or an electroless plating method for depositing nickel by a chemical reaction as a pretreatment.
Here, the surface of the master stamper 12 is transferred an uneven shape in an inverse pattern at the concave portion 10d on the surface of the master disk for mastering and formed with a convex portion 12p. 
When a plurality of stampers are necessary for an optical disk to be mass produced, a transfer process from a stamper to stamper by nickel plating is generally used as a method of obtaining a plurality of stampers from one master disk. Below, the method will be explained.
Namely, as shown in FIG. 3B, an oxide film 12a is formed on the obtained master stamper 12 by performing oxidization processing and, furthermore, plating processing with nickel or other material is performed to form a mother stamper 13. A plurality of mother stampers 13 can be formed from one master stamper 12.
Here, the surface of the mother stamper 13 is transferred an uneven shape in an inverse pattern at the convex portion 12p on the surface of the master stamper 12 and formed with a concave portion 13d. 
Next, as shown in FIG. 4A, an oxide film 13a is formed on the mother stamper 13 obtained above by performing oxidization processing and, furthermore, plating processing, etc. with nickel or other material is performed, so that a son stamper 14 is formed. The son stamper 14 has the same uneven pattern as that of the master stamper 12. A plurality of son stampers 14 can be formed from one mother stamper 13.
Here, the surface of the son stamper 14 is transferred an uneven shape in an inverse pattern at the concave portion 13d on the surface of the mother stamper 13 and formed with a convex portion 14p. 
Next, as shown in FIG. 4B, the son stamper 14 obtained above is set in a mold for injection molding, and a polycarbonate or other resin is injected to form a medium substrate 15 on the uneven pattern of the son stamper 14.
Here, the surface of the medium substrate 15 is transferred an uneven in an inverse pattern at the convex portion 14p on the surface of the son stamper 14 and formed with a concave portion 15d. 
When the number of optical disks to be formed is small, the master stamper 12 may be used instead of the son stamper 14 in this injection molding process to form the medium substrate 15.
Next, as shown in FIG. 5A, on the surface of the medium substrate 15, an optical recording layer 16 made by a stacked structure composed of a dielectric film, a recording film, a dielectric film and reflection film, etc. is formed in this order, for example, by a sputtering method, etc.
Next, as shown in FIG. 5B, a protective layer 17 is formed on the optical recording layer 16.
From the above, an optical disk as shown in FIG. 1 can be produced.
In the above production process, by forming a successive signal pit arrays having a length corresponding to recording data by performing exposure in the producing process of the master disk for mastering with a light modulated to have an intensity matching with a signal pit to form an uneven shape including the concave portion 15d of the medium substrate 15, and forming an optical recording layer by a reflection film, such as an aluminum film, a read-only-memory (ROM) type optical disk can be also produced.
The above optical disk is divided to a character portion CA for recording characters, such as a sequential number of the master disk for mastering, production date and product information, as shown in FIG. 6A, other than a signal portion SG, such as these pits and grooves, involved in recording/reproducing of information, within the same surface (normally on the inner circumferential (center hole CH) side of the signal portion SG).
FIG. 6B is a sectional view of a part corresponding to the signal portion SG and the character portion CA on the optical disk.
Characters are drawn in the character portion CA by filling inside of outlines of the characters with discontinuous groove (GR) arrays or pit arrays. Outside of the outlines of the characters is a non-recorded portion and has a mirror face (MR), so that it makes contrast and the characters become distinct to be able to be visually recognized as characters when looking from the medium substrate side, that is, from the Z-direction.
Also, inversely, inside of the characters may be a mirror face as a non-recorded portion and outside thereof may be filled with the grooves (GR) or pit arrays.
In the exposure process in forming a master disk for mastering, exposure is performed to form grooves or pit arrays inside the outlines of characters on the above character portion. Since an exposure apparatus to be used in the exposure step generally records in spirals, it disassembles the character string in the radius direction (the track pitch direction) and records only inside of the outlines of the characters by grooves or pit arrays for every track. When exposing an adjacent track after rotating the master disk for mastering once, it is necessary to adjust timing so that the previous track and characters are combined correctly.
When a user inputs desired characters to the exposure apparatus, a character producing device inside the exposure apparatus converts the character string to a recording signal in accordance with the above operation, and the recording signal is output to an optical modulator while synchronizing with a rotation cycle of the mastering master disk at the time of exposing.
A height of a character means a length in the vertical direction of the character, which is (recording track pitch)×(the number of tracks), and is normally 1 μm pitch×1000 tracks or so, and characters are drawn to be a height of 1 mm or so.
Here, in the conventional method, when exposing the characters on the master disk for mastering, “reversed characters” (that is, characters reversed by reflecting “normal characters” on a mirror) were used for recording. The reason will be explained below.
In a conventional CD and DVD format (including a write-once type, a rewritable type and other recordable types), a disk was produced by a method of transferring from a master stamper or a son stamper to a medium substrate made by a polycarbonate resin, etc., forming a reflection film or a recording film, etc. on the medium substrate, and applying a protective film thereon.
In such an optical disk, it is easier to read characters from the medium substrate side of plastic than to read from the protective layer side. It is because when a reflection film or a recording film is formed on a pattern, concave portions, such as grooves, are buried with the reflection film or the recording film and difference in level of the concave portions, such as grooves, decreases.
Also, there are the facts that since printing is performed on the protective layer in CDs, characters are erased in some cases, and DVDs are configured that plastic medium substrates having a thickness of 0.6 mm are put together on the film formation side of a reflection film or a recording film, so that the characters can be read only from the medium substrate side.
From the above reasons, the case of being “normal characters” when looking from the medium substrate side was dominant in conventional optical disks.
To be seen as “normal characters” from the medium substrate side as above, it was necessary to record as “reversed characters” when recording the characters on the master disk for mastering. The characters are reversed when transferred from a master disk for mastering to a master stamper, transferred from the master stamper to a mother stamper, transferred from the mother stamper to a son stamper, and transferred from a variety of stampers to a medium substrate in the mastering process. In the above production method of an optical disk according to the conventional example, since transferring to the medium substrate is performed from a son stamper or a master stamper, characters have to be “normal characters” on the son stamper or the master stamper (the master stamper and the son stamper have the same character direction). That is to say, it is necessary to record as “reversed characters” on the master disk for mastering.
In an optical disk as above, a recording/reproducing laser light is converged on the optical recording layer by an objective lens, and the smaller a diameter Ø of a light convergence spot becomes, the finer a pattern able to be recorded and reproduced becomes.
The diameter Ø of a light convergence spot is expressed by Ø=λ/NA from a wavelength λ of the recording/reproducing laser light and a numerical aperture NA of the objective lens, which indicates that a shorter wavelength of the recording/reproducing light or a higher numerical aperture contributes to higher density recording and a larger capacity.
For example, in CDs, the recording/reproducing laser light is 780 nm, the numerical aperture (NA) of the objective lens is 0.45, and a recording capacity is 650 MB, while in DVD-ROM (read-only memory), the laser light wavelength is 650 nm, the NA is 0.6 and the recording capacity is 4.7 GB.
Furthermore, as an optical disk system of the next generation, a development of an optical disk system has been pursued by using an optical disk of a type wherein a thin light transmitting layer of 0.1 mm or so is formed as a protective layer on the optical recording layer, the recording/reproducing light is irradiated to the optical recording layer through the protective layer, the laser light wavelength is 450 nm or less (for example 400 nm), the NA is 0.78 or more (for example 0.85), and the capacity is increased to, for example, five times as much as that of DVDs.
However, in the above large capacity optical disk, a method of transferring an uneven shape, such as grooves, from a mother stamper to a medium substrate has been examined, and when recording characters as “reversed characters” on the master disk as in the conventional example in such a production method, the characters becomes “normal characters” only when looking from the protective layer side having a thickness of 0.1 mm in the finally formed optical disk. However, in the above large capacity optical disk, grooves are made as shallow as 25 nm or less and a film thickness of a reflection film and recording film, etc, becomes thicker than the depth of the grooves, so that there is a disadvantage that it is very difficult to recognize the characters from the protective layer side having a thickness of 0.1 mm from which the characters can be read as “normal characters”.
Moreover, in the case of an optical disk having an optical recording layer including a phase change type recording film, initialization after film formation is necessary to make the optical recording layer be recordable. The initialization processing is performed on the signal portion for recording signals and normally not performed on the character portion.
However, the optical recording layer including a phase change type recording film not subjected to initialization processing has a low reflectance, and there is a disadvantage that characters become harder to be recognized by a character portion uneven shape.